Photothermographic film generally includes a base material coated on at least one side with an emulsion of heat sensitive materials. Once the film has been subjected to photo-stimulation by optical means (e.g., laser light), or “imaged”, the resulting latent image is developed through the application of heat to the film. In general, the uniformity in the density of a developed image is affected by the manner in which heat is transferred to the emulsion of heat sensitive material. During the developing process, uneven contact between the film and supporting structures can result in non-uniform heating of the film which, in-turn, can result in an uneven image density and other visual artifacts in the developed image. Therefore, the uniform transfer of heat to the heat sensitive materials during the developing process is critical in producing a high quality image.
Several types of thermal processing machines have been developed in efforts to achieve optimal heat transfer to sheets of photothermographic film during processing. One type of thermal processor, commonly referred to as a “flat bed” thermal processor, generally comprises an oven enclosure within which a number of evenly spaced rollers are configured so as to form a generally horizontal transport path through the oven. Some type of drive system is employed to cause the rollers to rotate, such that contact between the rollers and a piece of imaged film moves the film through the oven along the transport path from an oven entrance to an oven exit. As the film moves through the oven, it is heated to a required temperature for a required time period necessary to optimally develop the image.
While flat-bed type thermal processors are effective at developing photothermographic film, variations in image density can occur as the film moves through the oven. For instance, as a piece of film is transferred from one roller to the next, the lead edge can butt or “stub” into the next roller along the transport path until it eventually rides over the roller and is moved on to the next downstream roller. When the film stubs into a downstream roller, the force, although small, can be sufficient to cause a change in the velocity of the film as it moves along the transport path. Depending on the films rigidity, this velocity change may cause the film to either lift off from or to remain too long in contact with the surface of preceding rollers along the transport path and cause those areas of the film proximate to the roller surfaces to be heated differently than adjacent areas. A less rigid film may lift off from the roller surface and result in less heating to such areas than adjacent areas, while a more rigid film may remain for longer than a desired time on the roller surface and result in more heating to such areas than adjacent areas. In another instance, as the film moves along the transport path, the trailing edge may not maintain a desired contact with the roller surfaces and also in uneven heat transfer to the trailing edge.
Such non-uniform heating can produce variations in image density in the developed image which appear in the form of visible bands across the film. This effect is commonly referred to as “cross-width” or “cross-web” banding. Too much heating can result in “dark” bands, while too little heating may result in “light” bands. Furthermore, because the rollers are evenly spaced, the banding effect is reinforced at the same locations on the film as it moves from roller to roller along the transport path, and thus becomes increasingly visible as the film is processed.
Such cross-web banding is of particular concern in thermal processors employing heated rollers, such as that described by U.S. patent application Ser. No. 10/873,816 entitled “Flat Bed Thermal Processor Employing Heated Rollers”, (Kodak Docket No. 87968/SLP) filed on Jun. 22, 2004, assigned to the same assignee as the present application, and herein incorporated by reference. It is also more of a concern with rollers forming an initial portion of the transport path, as the difference in heat transfer to the film caused by its being lifted from or stalling on the roller surfaces is lessened as the film nears a desired developing temperature along the latter portions of the transport path.
It is evident that there is a continuing need for improved photothermographic film developers. In particular, there is a need for a flat bed type thermal processor having a roller system that substantially eliminates the above described cross-web banding effect.